System and method for calibrating a PET scanner

ABSTRACT

A method and system for calibrating a PET scanner are described. The PET scanner may have a field of view (FOV) and multiple detector rings. A detector ring may have multiple detector units. A line of response (LOR) connecting a first detector unit and a second detector unit of the PET scanner may be determined. The LOR may correlate to coincidence events resulting from annihilation of positrons emitted by a radiation source. A first time of flight (TOF) of the LOR may be calculated based on the coincidence events. The position of the radiation source may be determined. A second TOF of the LOR may be calculated based on the position of the radiation source. A time offset may be calculated based on the first TOF and the second TOF. The first detector unit and the second detector unit may be calibrated based on the time offset.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No.15/171,877, filed on Jun. 2, 2016, which claims priority of ChinesePatent Application No. 201510603207.2 filed on Sep. 21, 2015, ChinesePatent Application No. 201510854615.5 filed on Nov. 28, 2015, ChinesePatent Application No. 201511031899.4 filed on Dec. 31, 2015, andChinese Patent Application No. 201521140680.3 filed on Dec. 31, 2015,each of which is hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure generally relates to imaging, and moreparticularly, a system and method for TOF calibration in a PET scanner.

BACKGROUND

Positron emission tomography (PET) has been widely used in medicine fordiagnosis and other purposes. A subject, such as a patient, may bescanned with a PET scanner to obtain medical images. A PET scannerincludes a plurality of detector units. The detector units are used fordetecting coincidence events.

Time-of-flight (TOF) information is generally used for PET imagereconstruction. For an annihilation event, the time that each of thecoincident photons is detected at two detector units (or referred to asarrival time), and the difference is calculated. Since the traveldistances of the two photons to their respective detector units may bedifferent from each other, the photon whose travel distance is shortermay arrive at its detector unit first, compared to the other photon. Thedifference in the arrival time of the coincident photons may help pindown the location of the annihilation event along the line between thetwo detector units. Accurate TOF may allow the reconstruction of a PETimage.

In general, a phantom is used to calibrate and/or verify the accuracy ofa PET scanner. A phantom is a model with a known geometry (e.g., shape,size, etc.) and/or a known distribution of radiation activitiesthroughout the body of the phantom. By imaging the phantom, the accuracyof the imaging apparatus for a three-dimensional or two-dimensionalimage may be assessed and the settings of the PET scanner may beadjusted based on the phantom data. For instance, the time offset of adetector unit may be calibrated. A phantom may be designed to be a solidbody, and the phantom may be placed at the center of the FOV of the PETscanner for a calibration scan. Thus, a device to adjust the position ofthe phantom is needed. Furthermore, these requests make the phantomimaging process complicated and time-consuming. A method to determinethe time offset for detector units is described in this application.

SUMMARY

In a first aspect of the present disclosure, a method for calibrating aPET scanner is provided. The PET scanner has a field of view (FOV) and aplurality of detector rings. Each detector ring has a plurality ofdetector units. Each detector unit may have a plurality of crystalelements. The method may determining a line of response (LOR)correlating to a plurality of coincidence events. The LOR connects afirst detector unit and a second detector unit of the PET scanner. Themethod may also include calculating a first time of flight (TOF) of theLOR based on the plurality of coincidence events. The method may furtherinclude determining the position of the radiation source and calculatinga second TOF of the LOR based on the position of the radiation source.The method may include calculating a time offset based on the first TOFand the second TOF, and the first unit and the second unit may becalibrated based on the time offset. In some embodiments, the timeoffset may be due to a channel delay. In some embodiments, the first TOFmay be calculated based on a filter window.

In a second aspect of the present disclosure, a PET scanner is provided.The PET scanner has a plurality of detector rings, and each detectorring has a plurality of detector units. The PET scanner may include acoincidence event detection circuit for detecting coincidence eventsresulting from annihilation of positrons emitted by a radiation source.The PET scanner may also include a host computer that is configured todetermine an LOR connecting a first detector unit and a second detectorunit of the plurality of detector units and the LOR may correlate to aplurality of coincidence events, calculate a first TOF of the LOR basedon the plurality of coincidence events, determine the position of theradiation source, calculate a second TOF of the LOR based on theposition of the radiation source, calculate a time offset based on thefirst TOF and the second TOF, and calibrate the first detector unit andthe second detector unit based on the time offset.

In some embodiments, the position of the radiation source may beadjusted based on a target position. In some embodiments, the targetposition may include a target axial position and a targetcircumferential position. In some embodiments, the first TOF may be anaverage of each TOF of the plurality of coincidence events.

In some embodiments, the first TOF of the LOR may be calculated based onthe plurality of coincidence events. In some embodiments, a histogrammay be created based on TOFs of the plurality of coincidence events. Thetime value of the center of the histogram may be calculated. In someembodiments, a sinogram corresponding to the plurality of coincidenceevents may be created. A measurement TOF of the LOR may be calculatedbased on the sinogram. The first TOF may be assessed based on themeasurement TOF.

In some embodiments, the second TOF may be calculated based on theposition of the radiation source. An intersection portion of the LOR andthe radiation source may be determined. A center of the intersectionportion may be determined. The second TOF may be calculated based on thecoincidence event occurred in the center of the intersection portion. Insome embodiments, the radiation source may be wrapped by a phantom.

Additional features will be set forth in part in the description whichfollows, and in part will become apparent to those skilled in the artupon examination of the following and the accompanying drawings or maybe learned by production or operation of the examples. The features ofthe present disclosure may be realized and attained by practice or useof various aspects of the methodologies, instrumentalities andcombinations set forth in the detailed examples discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is further described in terms of exemplaryembodiments. These exemplary embodiments are described in detail withreference to the drawings. These embodiments are non-limiting examples,in which like reference numerals represent similar structures throughoutthe several views of the drawings, and wherein:

FIG. 1 illustrates an exemplary PET scanner according to someembodiments of the present disclosure;

FIG. 2 is a block diagram of an exemplary configuration engine accordingto some embodiments of the present disclosure;

FIG. 3 is a flowchart illustrating a process for the calibration of aPET scanner according to some embodiments of the present disclosure;

FIG. 4 illustrates an exemplary detector ring according to someembodiments of the present disclosure;

FIG. 5 is a block diagram of a TOF determination module according tosome embodiments of the present disclosure;

FIG. 6 illustrates a detection of photons in a PET scanner according tosome embodiments of the present disclosure;

FIG. 7 is a flowchart illustrating a process for TOF determinationaccording to some embodiments of the present disclosure;

FIG. 8 illustrates a location information of projection points accordingto some embodiments of the present disclosure;

FIG. 9 illustrates a location information of projection points accordingto some embodiments of the present disclosure;

FIG. 10 illustrates a block diagram of a quality determination moduleaccording to some embodiments of the present disclosure;

FIG. 11 is a flowchart illustrating a process for TOF qualitydetermination according to some embodiments of the present disclosure;

FIG. 12 illustrates an exemplary coordinate system according to someembodiments of the present disclosure;

FIG. 13 illustrates an exemplary coordinate system according to someembodiments of the present disclosure;

FIG. 14A and FIG. 14B illustrate two diagrams of TOF quality accordingto some embodiments of the present disclosure;

FIG. 15 illustrates an exemplary hollow cylindrical phantom according tosome embodiments of the present disclosure;

FIG. 16 is a flowchart illustrating a process for time calibrationaccording to some embodiments of the present disclosure;

FIG. 17 illustrates a histogram relating to LOR according to someembodiments of the present disclosure;

FIG. 18 is a block diagram of a radiation source adjustment moduleaccording to some embodiments of the present disclosure;

FIG. 19 is a flowchart illustrating a process for radiation sourceadjustment according to some embodiments of the present disclosure;

FIG. 20 is a flowchart illustrating a process for radiation sourceadjustment according to some embodiments of the present disclosure;

FIG. 21 illustrates an exemplary coordinate system for a sinogramaccording to some embodiments of the present disclosure;

FIG. 22A and FIG. 22B illustrate side view of an exemplary transportdevice according to some embodiments of the present disclosure;

FIG. 23A and FIG. 23B illustrate an exemplary transport device accordingto some embodiments of the present disclosure;

FIG. 24 illustrates an intersection of a radiation source and an LORaccording to some embodiments of the present disclosure;

FIG. 25 illustrates an exemplary detector ring according to someembodiments of the present disclosure;

FIG. 26 is a flowchart illustrating a process for time calibrationaccording to some embodiments of the present disclosure; and

FIG. 27 illustrates an exemplary coordinate system according to someembodiments of the present disclosure.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the claims.

It will be understood that when a module or unit is referred to as being“on,” “connected to,” or “coupled to” another module or unit, it may bedirectly on, connected or coupled to the other module or unit orintervening module or unit may be present. In contrast, when a module orunit is referred to as being “directly on,” “directly connected to” or“directly coupled to” another module or unit, there may be nointervening module or unit present. As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a”, “an” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

FIG. 1 is an exemplary PET scanner 100 according to some embodiments ofthe present disclosure. The PET scanner 100 may include a scanning bore110, a detector ring 120, a table 140, a coincidence event detectioncircuit 150, and a host computer 160.

In some embodiments, the scanning bore 110 may be configured to includethe detector ring 120 and be connected with the coincidence eventdetection circuit 150. As shown in the figure, a coordinate system maybe employed. The z-axis may denote the longitudinal axis of the scanningbore 110, and the plane defined by the x-axis and the y-axis, i.e., thex-y plane, may denote the cross section of the scanning bore 110 viewedalong the longitudinal axis, (which is also referred to as thelongitudinal section of the scanning bore 110). In some embodiments, thescanning bore 110 may include a plurality of detector rings, forexample, 96 detector rings.

The detector ring 120 may include a plurality of detector units, thedetector units may be implemented in any suitable manner, for example, aring, a rectangle, an array, etc. The detector ring 120 may be arrangedaround a detecting region 130 to detect radiation events (e.g., gammarays, coincidence events, photons, etc.) emitted from the detectingregion 130. In some embodiments, the detector units of the detector ring120 may be arranged in a number of detector rings (e.g., two, five, ten,a hundred, etc.) along an axial direction. In some embodiments, adetector unit of the detector ring 120 may include one or more crystalelements and/or one or more photomultiplier tubes (PMT) (not shown inthe figure).

In some embodiments, a PMT as employed in the present disclosure may bea single-channel PMT. In some embodiments, a PMT as employed in thepresent disclosure may be a multi-channel PMT.

The table 140 may be configured to position a patient or a subject inthe detecting region 130. In some embodiments, the table 140 may belinearly moved in an axial direction or the z-direction that istransverse to the detector ring 120 to facilitate the acquisition ofthree-dimensional (3D) data. In some embodiments, the table 140 may beused to adjust the position of a radiation source.

In some embodiments, a radiation source may be injected into a subject(e.g., a patient). The subject may be positioned in the detecting region130. The radiation source placed in the subject may undergo radioactivedecay, which may generate an emission of positrons. The positrons mayinteract with electrons nearby and start to annihilate. The annihilationmay produce two oppositely directed gamma photons. The two oppositelydirected gamma photons may strike the detector ring 120, e.g., thecrystal element(s) of the detector ring 120. The crystal element(s) mayproduce a scintillation of light when struck by the gamma photons. Thelight produced by the crystal element(s) may be received by one or morePMTs. The PMTs may be configured to convert the light into one or moreelectrical signals. The coincidence event detection circuit 150 may beconfigured to receive the electrical signals and provide signalamplification, filtering, conditioning, etc.

The coincidence event detection circuit 150 may include a converter (notshown in the figure) that may be used to digitize and time stamp theelectrical signals. The coincidence event detection circuit 150 may alsoinclude a pair detector (not shown in the figure) that may be used todetect and identify photon pair(s) belonging to a coincidence event. Theterm “photon pair” as used herein may refer to a pair of gamma photonsthat belong to a coincidence event resulting from a single annihilation.Upon identifying a photon pair, a line of response (LOR) processor (notshown in the figure) may process spatial information of the photon pairto identify an LOR connecting the two gamma photons. Since the two gammaphotons emitted in an annihilation event are spatially oppositelydirected, the annihilation event may be known to have occurred somewhereon the LOR. The detector ring 120 may have a sufficiently high temporalresolution to detect time-of-flight (TOF) between the two gamma photonsbelonging to a same coincidence event. A TOF processor (not shown in thefigure) of the coincidence event detection circuit 150 may analyze thetime difference between the arrival time of the two gamma photons tolocalize the position in which the annihilation event occurred along theLOR. The term “arrival time” as used herein may refer to the time aphoton strikes a detector and/or a crystal element of the PET scanner.In some embodiments, a TOF may be calculated by the TOF processor. Theterm “TOF” as used herein may refer to the time difference between thearrival time of two gamma photons striking on the detector ring 120, andthe two gamma photons belong to a same coincidence event.

As multiple coincidence events being accumulated in the PET scanner, aset of histoprojections may be generated. A reconstruction engine (notshown in the figure) of the coincidence event detection circuit 150 maybe used to reconstruct the set of histoprojections to generate one ormore images using a suitable reconstruction algorithm such as iterativebackprojection with correction, filtered backprojection, etc. The rawdata and/or the reconstructed images may be stored in a storage (notshown in the figure), and may be displayed, archived, processed,printed, filmed, transferred to another device, displayed on a display(not shown in the figure). A user including, for example, an operator,etc., may use the raw data and/or the reconstructed images to controlthe PET scanner 100, diagnose a subject, etc. In some embodiments, theuser may control the PET scanner 100 via the host computer 160.

In some embodiments, the PET scanner may include a configuration engine.In some embodiments, the configuration engine may be used to calculateTOF. In some embodiments, the configuration engine may be used tocalibrate TOF. In some embodiments, the configuration engine may be usedto assess the quality of TOF. In some embodiment, the configurationengine may be used to adjustment the position of a radiation source. Insome embodiments, the configuration engine may be located in the hostcomputer 160. In some embodiments, the configuration engine may be partof the coincidence event detection circuit 150.

The LOR processor and/or the TOF processor may include anyprocessor-based units and/or microprocessor-based units. Merely by wayof example, the units may include a microcontroller, a reducedinstruction set computer (RISC), application specific integratedcircuits (ASICs), an application-specific instruction-set processor(ASIP), a central processing unit (CPU), a graphics processing unit(GPU), a physics processing unit (PPU), a microcontroller unit, adigital signal processor (DSP), a field programmable gate array (FPGA),an acorn reduced instruction set computing (RISC) machine (ARM), or anyother circuit or processor capable of executing the functions describedherein, or the like, or any combination thereof. The exemplary types ofprocessors that may be used in connection with the present system arenot exhaustive and are not limiting. After consulting the presentdisclosure, one skilled in the art may envisage numerous other changes,substitutions, variations, alterations, and modifications withoutinventive activity, and it is intended that the present disclosureencompasses all such changes, substitutions, variations, alterations,and modifications as falling within its scope.

It should be noted that the description of the PET scanner is providedfor the purposes of illustration, and not intended to limit the scope ofthe present disclosure. For persons having ordinary skills in the art,various modifications and variations may be conducted under the teachingof the present disclosure. However, those modifications and variationsmay still pertain to the present disclosure. For example, the PETscanner as described above may employ techniques including digitalsubtraction angiography (DSA), computed tomography (CT), computedtomography angiography (CTA), positron emission tomography (PET), X-rayimaging, magnetic resonance imaging (MRI), magnetic resonanceangiography (MRA), single-photon emission computerized tomography(SPECT), ultrasound scanning (US), CT-MR, CT-PET, CE-SPECT, DSA-MR,PET-MR, PET-US, SPECT-US, transcranial magnetic stimulation(TMS)-MR,US-CT, US-MR, X-ray-CT, X-ray-MR, X-ray-portal, X-ray-US, Video-CT, andVide-US. In some embodiments, the radiation source described above mayinclude a phantom for testing the performance of the PET scanner.

FIG. 2 is a block diagram of an exemplary configuration engine 200implemented in the PET scanner 100 according to some embodiments of thepresent disclosure. As shown in the figure, the configuration engine 200may include a TOF determination module 201, a calibration module 202, aquality determination module 203, and a radiation source adjustmentmodule 204.

The TOF determination module 201 may be configured to calculate a TOF ofa coincidence event. In some embodiments, the TOF determination module201 may be used to calculate the TOF of an LOR.

The calibration module 202 may be configured to calibrate the PETscanner as described in connection with FIG. 1 . In some embodiments,the calibration module 202 may be configured to calibrate the crystalelements of the PET scanner. In some embodiments, the calibration module202 may be configured to calibrate the photomultiplier tubes of the PETscanner.

The quality determination module 203 may be configured to determine thequality of the TOF calculated by the TOF determination module 201. Insome embodiments, the TOF calculated by the TOF determination module 201may be assessed based on one or more measurement TOFs and/or thresholds.

The radiation source adjustment module 204 may be configured to adjustthe position of a radiation source placed in the scanning region of thePET scanner. In some embodiments, the radiation source adjustment module204 may be configured to adjust the position of a radiation source basedon a target position.

It should be noted that the configuration engine described above isprovided for the purposes of illustration, and not intended to limit thescope of the present disclosure. For persons having ordinary skills inthe art, various modifications and variations may be conducted under theteachings of the present disclosure. However, those modifications andvariations may not depart from the scope of the present disclosure.

FIG. 3 is a flowchart illustrating a process for TOF calibrationaccording to some embodiments of the present disclosure.

In step 301, a radiation source may be placed in a PET scanner. In someembodiments, the radiation source may be placed in the scanning bore 110of the PET scanner. In some embodiments, the radiation source may beplaced in the center of the scanning bore 110. In some embodiments, theradiation source may be placed in the peripheral part of the center ofthe scanning bore 110.

In step 303, TOF may be acquired. In some embodiments, the TOF of acoincidence event may be acquired. In some embodiments, the TOF of anLOR may be acquired. In some embodiments, a plurality of coincidenceevents may correlate to the LOR, and each coincidence event may have anLOR. The TOF of the LOR may be calculated based on the plurality of LORsof the plurality of coincidence events. For example, the TOF of the LORmay be an average of the plurality TOFs. In some embodiments, ahistogram may be created based on the plurality of TOFs, the TOF of theLOR may be calculated based on the histogram.

In step 305, the quality of TOF may be determine. In some embodiments,the TOF acquired in step 303 may be assessed in step 305. A TOF may bediscarded if the assessment reveals that the TOF is unsatisfactory (orreferred to as “fail”), and the TOF may be validated if the assessmentreveals that the TOF is satisfactory (or referred as to “succeed”). Insome embodiments, a measurement TOF may be calculated, and theassessment of the TOF may be based on the measurement TOF.

In step 307, the PET scanner may be calibrated based on TOF. In someembodiments, a time offset table may be prestored in or accessible fromthe PET scanner. TOF calculated by the PET scanner may be calibratedbased on the time offset table.

It should be noted that the flowchart is provided for the purposes ofillustration, and not intended to limit the scope of the presentdisclosure. In some embodiments, step 305 may be skipped, and step 303may proceed to step 307 directly.

FIG. 4 illustrates an exemplary detector ring 400 according to someembodiments of the present disclosure. As shown in the figure, thedetector ring 400 may include a plurality of detector units arranged ina ring. A radiation source 401 may be placed in the detector ring 400.The radiation source 401 may decay and generate an emission ofpositrons. The positrons may interact with electrons nearby andannihilation may occur. The annihilation may produce two gamma photonsthat may strike the detector ring 400, e.g., one or more detector unitsof the detector ring 400. A spatial line indicating the path in whichthe gamma photons direct may be generated, the spatial line may betermed as a LOR 402. The LOR 402 may indicate the gamma photonsgenerated by the annihilation of the radiation source 401.

FIG. 5 is a block diagram of an exemplary TOF determination module 201according to some embodiments of the present disclosure. The TOFdetermination module 201 may designate a first photon and a secondphoton as a photon pair based on the time and location informationrelating to the first photon and the second photon. The term “photonpair” as used herein may refer to two photons originated from a commonannihilation event and generate a coincidence event on the PET scanner.The second photon may be assessed based on a filter window. If theassessment succeeds, the first photon and the second photon may bedesignated as a photon pair. Otherwise, the second photon may bediscarded. After a photon pair is designated, the TOF of the photon pairmay be calculated based on the arrival time of photons of the photonpair. In some embodiments, the TOF determination module 201 may includea time information acquisition unit 501, a first determination unit 502,a second determination unit 503, a judgment unit 504, and a pairing unit505.

The time information acquisition unit 501 may be configured to acquirethe time information of the photons. In some embodiments, the timeinformation acquisition unit 501 may acquire the arrival times of afirst photon and a second photon that may constitute a coincident photonpair. In some embodiments, a first photon and a second photon of acoincidence event may strike the detector units as described inconnection with FIG. 1 . The arrival times of the first photon and thesecond photon may be recorded by the coincidence event detection circuit150.

The first determination unit 502 and the second determination unit 503may be configured to determine the location information of the firstphoton and the second photon. In some embodiments, the firstdetermination unit 502 may be configured to determine the projectionpoint of the first photon in the axial direction of the detector ring120 and/or the scanning bore 110 as a first projection point. The axialdirection of the detector ring 120 and the axial direction of thescanning bore 110 may be essentially the same or essentially parallel toeach other. The axis of the detector ring 120 may essentially coincidewith or be essentially parallel to the axis of the scanning bore 110.The first determination unit 502 may be configured to determine theprojection point of the second photon in the axial direction of thedetector ring 120 and/or the scanning bore 110 as a second projectionpoint. As used herein, “essentially,” as in “essentially the same,”“essentially coincide with,” “essentially parallel to,” etc., withrespect to a parameter or a characteristic may indicate that thevariation is within 2%, or 5%, or 8%, or 10%, or 15%, or 20% of theparameter or the characteristic, or an average value of the parameterin, for example, a detector or a PET scanner, etc.

In some embodiments, the second determination unit 503 may be configuredto determine the projection point of the first photon in thecircumferential direction of the detector ring 120 and/or the scanningbore 110 as a third projection point. The circumferential direction ofthe detector ring 120 and the circumferential direction of the scanningbore 110 may be essentially the same or essentially parallel to eachother. The second determination unit 503 may be configured to determinethe projection point of the second photon in the circumferentialdirection of the detector ring 120 and/or the scanning bore 110 as afourth projection point.

In some embodiments, the first determination unit 502 and the seconddetermination unit 503 may be combined as a single unit.

The judgment unit 504 may be configured to determine whether the arrivaltime of the second photon falls within the filter window from thearrival time of the first photon. In some embodiments, the judgment unit504 may judge whether the distance between the first projection pointand the second projection point is less than or equal to a firstthreshold. The judgment unit 504 may also judge whether the arc lengthbetween the third projection point and the symmetry point of the fourthprojection point is less than or equal to a second threshold. A symmetrypoint is described elsewhere in the present disclosure. See, forexample, FIGS. 7 and 9 and the description thereof. The first thresholdand the second threshold may relate to the size of the filter window. Insome embodiments, the first threshold and the second threshold may beselected based on the size of the filter window. For instance, the sizeof the filter window may be 11*11 width of the crystals; the firstthreshold and the second threshold may be set to be the width of 5crystals.

In some embodiments, the pairing unit 505 may designate the first photonand the second photon as a photon pair. The TOF of the coincidence eventtriggered by the first photon and the second photon may be determinedbased on the information of the photon pair. In some embodiments, if theresult of the judgment unit 504 validates the second photon, the firstphoton and the second photon may be paired. Otherwise, the second photonmay be discarded.

FIG. 6 illustrates a detection of photons in the PET scanner accordingto some embodiments of the present disclosure. As shown in the figure, acoincident photon pair (photon P1 and photon P2) generated byannihilation events may be detected by the PET scanner 100. The term“coincident photon pair” as used herein may refer to two photonsgenerated from one or more annihilation event. A filter window 602 maybe used to determine whether photon P1 and photon P2 are generated in asame annihilation event based on their positions. In some embodiments,if photon P2 falls into the filter window 602, photon P1 and photon P2may be designated as a photon pair originated from a same annihilationevent. Otherwise, photon P1 and photon P2 may not be designated as aphoton pair originated from a same annihilation event, and photon P2 maybe discarded.

In some embodiments, the filter window 602 may include a plurality ofdetector units of the PET scanner 100. In some embodiments, the filterwindow 602 may include a plurality of crystal elements (or referred asto crystals for brevity) of the PET scanner 100. In some embodiments,the size of the filter window may be determined by the number ofdetector rings and/or the number of crystal elements of the PET scanner100. For example, the filter window 602 may include a plurality ofcrystal elements stretching across one or more detector rings.

In some embodiments, circumference 603 may denote the circumference inwhich photon P1 is detected in the PET scanner 100, and O may denote thecenter of the circumference 603. P3 may denote the intersection pointbetween the circumference 603 and the straight line determined by O andphoton P1. In some embodiments, the position of the filter window 602may be determined based on P3. For instance, the center of the filterwindow 602 may be determined based on P3.

In some embodiments, P3 being designated as the center of the filterwindow 602 may be symmetric to P1 with respect to the longitudinalcenter line of the PET scanner 100.

FIG. 7 illustrates a flowchart of a process for TOF determination in thePET scanner according to some embodiments of the present disclosure. Instep 701, a filter window may be set. In some embodiments, the size ofthe filter window may be determined by the number of detector rings andthe number of crystal elements of the PET scanner. For example, the PETscanner may include 112 detector rings and 112×1152 crystal elementsdistributed in those 112 detector rings. The filter window may be aportion of the surface of the scanning bore of the PET scanner. Thefilter window may have a longitudinal width and a circumferential width.For instance, the longitudinal width may be the width of 11 crystalelements, and the circumferential width may be the width of 11 crystalelements as well.

In step 703, the arrival time and location information of a coincidentphoton pair may be received. In some embodiments, the time and locationinformation of the coincident photon pair may be acquired by thecoincidence event detection circuit 150 as described in connection withFIG. 1 . In some embodiments, the time and location information may bestored in the storage of the PET scanner, and fetched out for TOFcalculation. Two photons may be designated as a photon pair based ontheir arrival time information. In some embodiments, one photon of thecoincident photon pair is designated as a first photon and the other oneis designated as the candidate photon to be assessed. In someembodiments, the coincident photon pair may contain P1 and P2.

In step 705, the location information of the projection points of twophotons of a candidate coincident photon pair in the axial direction ofthe PET scanner may be received. In some embodiments, P1 may be thefirst photon generated in an annihilation event, while P2 may be acandidate photon to be assessed to determine whether P1 and P2 originatefrom a same annihilation event. In some embodiments, the firstprojection point may be the projection point of P1 in the axialdirection of the PET scanner. The second projection point may be theprojection point of P2 in the axial direction of the PET scanner.

FIG. 8 illustrates the location information of the first projectionpoint P1′ and the second projection point P2′. P1′ may denote the firstprojection point and P2′ may denote the second projection point. d2 maydenote the distance between P1′ and P2′.

Returning to FIG. 7 , in step 707, the distance between the firstprojection point and the second projection point may be assessed basedon a first threshold. If the distance between the first projection pointand the second projection point (e.g., d2 as illustrated in FIG. 8 ) isless than or equal to the first threshold, P2 may be further assessed.Otherwise, P2 may be discarded as P1 and P2 are deemed not originatedfrom a same annihilation event.

In some embodiments, the first threshold may correlate with the size ofthe filter window as described above. In some embodiments, thelongitudinal width of the filter window may be the width of m crystalelements, the first threshold may be set as the width of (m−1)/2 crystalelements, wherein m is an odd number. In some embodiment, the firstthreshold may be the width of 5 crystal elements. In some embodiments,the first threshold may be an even number. It should be noted that thefirst threshold may be variable and/or adjustable based on thelongitudinal width of the filter window. For example, the firstthreshold may be the width of m crystal elements, the width of (m−1)/acrystal elements, the width of m/a crystal elements, etc., and a maydenote an integer.

In step 709, the location information of the projection points of thecandidate coincident photon pair in the circumferential direction of thePET scanner may be received. In some embodiments, the third projectionpoint may be the projection point of P1 in the circumferential directionof the PET scanner. The fourth projection point may be the projectionpoint of P2 in the circumferential direction of the PET scanner.

FIG. 9 illustrates the location information of the third projectionpoint P1″, the fourth projection point P2″, and the symmetry point ofthe fourth projection point P2″′. d1 may denote the arc length betweenP1″ and P2″′.

Returning to FIG. 7 , in step 711, the distance between P1 and P2 alongthe circumferential direction may be assess in various ways. In someembodiments, the arc length between the third projection point and thesymmetry point of the fourth projection point may be assessed. In someembodiments, the arc length between the third projection point and thesymmetry point of the fourth projection point may be calculated. If thearc length between the third projection point and the symmetry point ofthe fourth projection point is less than or equal to a second threshold,the process may proceed further. Otherwise, P2 may be discarded. In someembodiments, the length of the line segment between the third projectionpoint and the symmetry point of the fourth projection point may becalculated. If the length of the line segment between the thirdprojection point and the symmetry point of the fourth projection pointis less than or equal to a second threshold, the process may proceedfurther. Otherwise, P2 may discarded. In some embodiments, the length ofthe line segment between the third projection point and the fourthprojection point may be calculated. If the difference of the length ofthe line segment between the third projection point and the fourthprojection point, and the diameter of the detector ring is less than orequal to a second threshold, the process may proceed further. Otherwise,P2 may be discarded.

In some embodiments, the second threshold may correlate to the size ofthe filter window. In some embodiments, the circumferential width of thefilter window may be the width of n crystal elements, the secondthreshold may be set as the width of (n−1)/2 crystal elements, wherein nis an odd number. In some embodiment, the second threshold may be thewidth of 5 crystal elements. In some embodiments, the second thresholdmay be an even number. It should be noted that the second threshold maybe variable and adjustable based on the circumferential width of thefilter window. For example, the second threshold may be the width of ncrystal elements, the width of (n−1)/b crystal elements, the width ofn/b crystal elements, etc., and b may denote an integer.

After the assessment in step 707 and step 711, the location of thesecond photon may be determined within the filter window correspondingto the first photon. Thus in step 713, P1 and P2 are designated as aphoton pair originated from a common annihilation event. In someembodiments, since the distance between the first projection point andthe second projection point is less than or equal to a first threshold,and the arc length between the third projection point and the symmetrypoint of the fourth projection point is less than or equal to a secondthreshold, the location of P2 may be determined within the filter windowcorresponding the location of P1. Thus P1 and P2 may be designated as aphoton pair originated from a common annihilation event.

In step 715, the TOF of the photon pair may be determined. The TOF ofthe photon pair may be determined based on the arrival time informationof photons of the photon pair.

In some embodiments, the process for TOF determination above may be usedto calculate the average TOF of a crystal element. In some embodiment,all photon pairs of a crystal element may be designated based on theprocess for TOF determination above. The sum of photon pairs of thecrystal element may be calculated. The average TOF may be the ratio ofthe sum of all TOF and the number of photon pairs. In some embodiments,the average TOF of a crystal element may be calculated based on:

$\begin{matrix}{{\overset{¯}{t_{J}} = \frac{\sum\limits_{i = 1}^{sum}\left( {t_{ji} - t_{i}} \right)}{sum}},} & \left( {{Equation}1} \right)\end{matrix}$in which j may denote the index number of a crystal elements, i maydenote the index number of the photon pair at crystal element j, t _(j)may be the average TOF of crystal element j, t_(ji) may denote the timeinformation (e.g., arrival time) of the first photon of the i^(th)photon pair at crystal element j, t_(i) may be the time information(e.g., arrival time) of the second photon of the i^(th) photon pair atcrystal element j, and sum may denote total number of photon pairsdetected by crystal element j. For instance, the PET scanner includes112 detector rings and 112×1152 crystal elements distributed in thosedetector rings; each detector ring includes 1152 crystal elements; itmay cost 0.561 s to process 9,000,000 coincident photon pairs using apersonal computer (PC) with a CPU clock speed of 3.1 GHz and a memory(e.g., ROM) of 4G.

FIG. 10 illustrates a block diagram of the quality determination module203 according to some embodiments of the present disclosure. The qualitydetermination module 203 may include a phantom position calculation unit1001, a TOF calculation unit 1002, a measurement unit 1003, and amatching unit 1004.

As shown in FIG. 10 , the phantom position calculation unit 1001 may beconfigured to calculate the location of a phantom. In some embodiments,the phantom may include a radiation source having a shape of a solidrod, a linear radiation source, a radiation source having a shape of acylinder, or the like, or any combination thereof. The phantom may belocated in the field of view (FOV) of the PET scanner and need not becoaxial or concentric with the scanning bore. In some embodiments, anaxis (e.g., the longitudinal axis) of the phantom may be parallel to thelongitudinal axis of the PET scanner.

The TOF calculation unit 1002 may be configured to calculate a first TOFof an LOR based on the position of the phantom.

The measurement unit 1003 may be configured to calculate a second TOFbased on the sinogram of LORs and a time offset table. The time offsettable may be prestored in or accessible to the PET scanner. The timeoffset table may be calibrated based on the sinogram of LORs.

The matching unit 1004 may be configured to assess the first TOF basedon the second TOF, and determined the quality of the first TOF. In someembodiments, the assessment may be based on the difference between thefirst TOF and the second TOF. The difference may be assessed based on athreshold. If the assessment succeeds (e.g., the difference does notexceed the threshold), the first TOF may be validated; otherwise, thefirst TOF may be invalidated and discarded.

FIG. 11 is a flowchart illustrating a process for TOF qualitydetermination according to some embodiments of the present disclosure.

In step 1101, a phantom may be placed in the FOV of the PET scanner, theaxis of the phantom may be parallel to the longitudinal axis of the PETscanner. In some embodiments, the phantom may be placed in any suitableposition in the PET scanner as long as it is located in the FOV of thePET scanner. In some embodiments, the phantom may be concentric with thescanning bore 110 of the PET scanner.

In step 1102, the position of the phantom may be calculated based on thecoincidence events detected by the PET scanner. FIG. 12 illustrates anexemplary coordinate system employed in the scanning bore of the PETscanner according to some embodiments of the present disclosure. Thecoordinate system may be used to determine the position of the phantom.O may denote the origin of the coordinate system. In some embodiments, Omay be the center of the scanning bore. The z-axis may denote thelongitudinal axis of the scanning bore 1200. Section 1201 may be acircle indicating the longitudinal section of the scanning bore, and theradius of the section 1201 may be denoted by r. In some embodiments, thesection 1201 may be a detector ring. The scanning bore 1200 may includea plurality of detector rings configured to detect gamma photonsgenerated by annihilation events. In some embodiments, a detector ringmay include a plurality of detector units including crystal elementsand/or crystal element array(s) the term “crystal element array” as usedherein may refer to an array of crystal elements.

In some embodiments, a detector ring may include a plurality of crystalelements. For example, two photons of a coincidence event may bedetected by a crystal element A(X_(a), Y_(a)) and a crystal elementB(X_(b), y_(b)), respectively. The line segment connecting crystalelement A(X_(a), Y_(a)) and crystal element B(X_(b), Y_(b)) may be theLOR of the coincidence event. In some embodiments, the LOR processor asdescribed in connection with FIG. 1 may be used to determine the LOR. Insome embodiments, the phantom position calculation unit 1001 asdescribed in connection with FIG. 10 may be configured to calculate theposition of the phantom on the LOR based on the arrival time of the twophotons striking the two crystal elements. The position of the phantommay be denoted as (X₀, Y₀), wherein 0<X₀<r and 0<Y₀<r. In someembodiments, the TOF processor as described in connection with FIG. 1may calculate the TOF of the coincidence event. The TOF may be used todetermine the position of the phantom.

In step 1103, a first TOF of the LOR may be calculated based on theposition of the phantom calculated in step 1102. Referring to FIG. 12 ,two photons originated from a same annihilation event occurred in thephantom C may strike crystal element A and crystal element B to generatea coincidence event. LOR 1202 (line segment AB) may denote the LOR ofthe coincidence event, and D may denote the center of AB. The x-y planemay denote the longitudinal section of the scanning bore 1200. Thez-axis may denote the longitudinal axis of the scanning bore 1200. Thez-axis may be perpendicular to the x-y plane. The distance between C andD may be calculated by:Δl=y ₀ cos φ−x ₀ sin φ,(−π≤φ≤+π),  (Equation 2)in which φ may denote the included angle of the LOR 1202 and y-axis, andΔl may denote the distance between C and D. Therefore, the difference ofline segment BC (the distance of the phantom C and the crystal elementB) and line segment AC (the distance of the phantom C and the crystalelement A) may be calculated by:Δs=2Δl=2(Δl=y ₀ cos φ−x ₀ sin φ),  (Equation 3)in which Δs may denote the difference of line segment BC and linesegment AC. Therefore, the TOF of the LOR 1202 may be calculated by:

$\begin{matrix}{{{\Delta t} = \frac{2\left( {{\gamma_{0}\cos\varphi} - {x_{0}\sin\varphi}} \right)}{c}},} & \left( {{Equation}4} \right)\end{matrix}$in which Δt may denote the first TOF of the LOR 1202, and c may denotethe light speed.

In some embodiments, Equation 2, Equation 3, and Equation 4 may be usedto calculate first TOF of a plurality of LORs generated by the phantom,and a curve of Δt may be generated based on the first TOF of the LORs.In some embodiments, Equation 2, Equation 3, and Equation 4 may be usedto calculate first TOFs of at least some LORs generated by the phantom,and a curve of Δt may be generated based on the first TOFs of theseLORs.

In step 1104, a second TOF of an LOR may be calculated based on asinogram and a time offset table.

In some embodiments, a time offset table may be prestored in the storageof the PET scanner or otherwise accessible by the PET scanner. The timeoffset table may be used to improve the resolution of the PET scannerfor the calculation of a TOF. Referring to FIG. 12 , the measurement TOFof an coincidence event may be calculated by:Δτ_(AB)=(T _(A) −OT _(A))−(T _(B) −OT _(B))=(T _(A) −T _(B))−(OT _(A)−OT _(B)),  (Equation 5)in which OT_(A) may denote the time offset of crystal element A, andOT_(B) may denote the time offset of crystal element B. (OT_(A)−OT_(B))may be calculated by looking up the time offset table. T_(A) may denotethe arrival time at crystal element A, while T_(B) may denote thearrival time at crystal element B. The time offset table may be indexedaccording to the index number of the crystal element. For example,crystal element A may have a time offset in the time offset table, andthe index number of the time offset may be denoted by A.

In some embodiments, the PET scanner may acquire a plurality of LORs indifferent angles, and create a sinogram of these LORs. In someembodiments, the time offset table may be calibrated based on thesinogram.

In some embodiments, a histogram of a plurality of LORs of the PETscanner may be created. The second TOF may be calculated based on thehistogram. The second TOF of an LOR may be calculated by:

$\begin{matrix}{{{\Delta{t'}} = \frac{\underset{i}{\Sigma}{\Delta\tau}_{i}n_{i}}{\underset{i}{\Sigma}n_{i}}},} & \left( {{Equation}6} \right)\end{matrix}$in which the bin of the histogram may cover the range from 5 picosecondsto 15 picoseconds. Δt′ may denote the second TOF. In some embodiments,the bin may be 10 picoseconds. In Equation 6, i may denote the indexnumber of histogram, n_(i) may denote the number of coincidence eventsin the i^(th) histogram, and Δτ_(i) may denote the measured TOF ofcoincidence events in the histogram.

FIG. 13 illustrates an exemplary coordinate system that may be employedin the scanning bore according to some embodiments of the presentdisclosure. As shown in the figure, a TOS coordinate systemcorresponding to an LOR ab may be created on a xOy plane. The t-axis maybe parallel to ab, and the s-axis may be perpendicular to ab andintersect with ab at the center point of ab. (r_(a), r_(b), φ_(j),s_(j)) may denote an LOR in the sinogram, in which j may denote theindex number of the LOR, (j=1, 2, 3 . . . 576), φ_(j) may denote theincluded angle between the LOR and the y-axis, and s_(j) may denote thedistance between the LOR and the center of the scanner bore (O as shownin the figure). ra and rb may denote the two crystal elementscorresponding to the LOR, respectively. In some embodiments, a pluralityof LORs may correspond to a pair of crystal elements. In someembodiments, a curve of Δt′ may be generated according to Equation 6.

In step 1105, the curve of Δt generated according to Equation 4 may beassessed based on the curve of Δt′ to determine the quality of the firstTOF. The curve of Δt may include a plurality of first TOFs of LORs,while the curve of Δt′ may include a plurality of second TOFs of LORs. Athreshold may be set to assess a first TOF based on the correspondingsecond TOF for a same LOR. If the difference between the first TOF andthe corresponding second TOF exceeds the threshold, the first TOF may bevalidated, otherwise, the first TOF may be invalidated and discarded.

FIG. 14A illustrates a diagram of validated first TOFs according to someembodiments of the present disclosure. The horizontal axis denotes theindex number of LOR. The vertical axis may denote a TOF of an LOR. Thesmooth curve 1401 may indicate first TOFs, while the coarse curve 1403may indicate second TOFs. Merely by way of example, the threshold may beset to be 25 picoseconds. The smooth curve 1401 and the coarse curve1403 may be assessed by:|Δ−Δt′|≤25 picoseconds.  (Equation 7)

If the absolute value of the difference between a first TOF and thecorresponding second TOF is less than or equal to 25 picoseconds, thefirst TOF is validated.

FIG. 14B illustrates a diagram of invalidated first TOFs according tosome embodiments of the present disclosure. As shown in the figure,curve P1 denotes first TOFs, while curve P2 denotes second TOFs. It maybe seen from the figure that the deviation of curve P2 from curve P1 islarger than that illustrated in FIG. 14A. In some embodiments, the firstTOFs in FIG. 14B may be invalidated.

In some embodiments, the phantom may be placed in the center of thescanning bore, e.g., being concentric with the scanning bore. In someembodiments, the phantom may not be concentric with the scanning bore.In some embodiments, the PET scanner may further include a calibrationmodule configured to calibrate first TOFs when the first TOF isinvalidated based on corresponding second TOFs. For example, if a firstTOF is invalidated, the value of the first TOF may be reassigned basedon the value of the corresponding second TOF.

FIG. 15 illustrates an exemplary hollow cylindrical phantom according tosome embodiments of the present disclosure. The time calibration of aplurality of detector units in a PET scanner may be performed using thephantom. In some embodiments, a first crystal element 1502 and multiplesecond crystal elements 1503 may be set inside a cylindrical bore 1501.In some embodiments, the first crystal element 1502 and multiple secondcrystal elements 1503 may form multiple crystal element pairs. A crystalelement pair may include the first crystal element 1502 and a secondcrystal element 1503 of the multiple second crystal elements. The firstcrystal element 1502 and the second crystal element 1503 may receive aphoton of a coincidence event, respectively. A hollow cylindricalphantom 1504 filled with a radiation source that may be placed insidethe bore 1501. Annihilation events may occur in the hollow cylindricalphantom 1504. In some embodiments, the center of symmetry of the phantommay be placed at the center of the FOV of the PET scanner and the centeraxis of the phantom 1504 and that of the bore 1501 may coincide. In someembodiments, the center of symmetry of the phantom may be placed not atthe center of the FOV of the PET scanner. In some embodiments, the shapeof the phantom 1504 may be symmetric. In some embodiments, the shape ofthe phantom may be asymmetric. In some embodiment, the thickness of theportion of the phantom 1504 filled with the radiation source may beuniform. In some embodiment, the thickness the portion of the phantom1504 filled with the radiation source may be non-uniform. The size ofthe phantom 1504 may correlate to the size of the FOV of the PETscanner. In some embodiments, the diameter of the phantom 1504 may rangefrom D_(FOV)/2 to D_(FOV), where D_(FOV) may denote the length of theFOV in the radial direction. In some embodiments, the phantom length inthe axial direction may be equal to or larger than the length of the FOVin the axial direction.

FIG. 16 is a flowchart illustrating a process for time calibration forthe PET scanner according to some embodiments of the present disclosure.In the process the TOF of the two photons in a coincidence event iscalculated. The process may be repeatedly performed to calibrate everycrystal element in the PET scanner. Thus the time offset for multiplecrystal elements may be determined. In step 1601, a hollow cylindricalphantom may be placed at the center of the FOV of the PET scanner. Insome embodiments, the time offset of the PET scanner may be cleared. Insome embodiments, all the electrical time offset of the PET scanner maybe cleared.

In step 1603, a first crystal element may be selected from a detectorring of the PET scanner. The first crystal element may be selected forthe calibration of its time offset, and n second crystal elements may beselected from the detector ring of the PET scanner to form n crystalelement pairs with the first detector unit. In some embodiments, n maydenote an integer. In some embodiments, n LORs may be determined basedon the first crystal element and the n second crystal elements.

In some embodiments, A may denote a first crystal element, and B_(i) maydenote a second crystal element that may detect a coincidence event withA. i may denote the index number of the second crystal element, and imay range from 1 to n. In some embodiments, LOR₁ may denote the LORbetween the first crystal element A and the second crystal element B₁.LOR₂ may denote the LOR between the first crystal element A and thesecond crystal element B₂. Likewise, LOR_(n) may denote the LOR betweenthe first crystal element A and the second crystal element B_(n). Insome embodiments, the LORs of the first crystal element and n secondcrystal elements B₁ through B_(n) may form a sector area 1505 as shownin FIG. 15 . In some embodiments, the LORs may pass through the hollowcylindrical phantom 1504 filled with a radiation source. The relativelocation of the first crystal element and the corresponding secondcrystal element may be determined based on the dimension of thereconstructed PET image. For a two-dimensional image reconstruction, thefirst crystal element and the multiple second crystal elements may belocated in a same detector ring. For a three-dimensional imagereconstruction, the first crystal element and the multiple secondcrystal elements may be located in a same detector ring or differentdetector rings. For example, the first crystal element and the multiplesecond crystal elements may be located in different detector rings.

In step 1605, the TOF of a coincident photon pair detected between thefirst crystal element and a corresponding second crystal element may becalculated. In some embodiments, m coincidence events may be detected bythe first crystal element and the corresponding second crystal element.Therefore, m TOFs may be acquired. A histogram of the m TOFs may becreated (FIG. 17 ). The n crystal element pairs may receive coincidentphoton pairs and their time information.

FIG. 17 illustrates a histogram relating to the first crystal elementand the corresponding second crystal element according to someembodiments of the present disclosure. The horizontal axis of thehistogram may denote time the accumulation of coincidence events, andthe bin width may be set to 5 picoseconds. In some embodiments, the binwidth may be defined by a user. The vertical axis of the histogram maydenote the number of coincidence events within a certain period of time.The histogram as illustrated in FIG. 17 may be symmetric and have twopeaks.

In some embodiments, the histogram may be axisymmetric. In someembodiments, the center of the symmetry of the histogram may be T=0. Insome embodiments, the center of the symmetry of the histogram may not beT=0. In some embodiment, the histogram may be nonaxisymmetric. In someembodiments, the crest factor of the two peaks may be different.

In step 1607, a time value T_(c) may be calculated. In some embodiments,Tc may be the time value of the center of the symmetry of the histogram.In some embodiments, T_(c) may be calculated by:

$\begin{matrix}{{T_{c} = {{\frac{1}{m}{\sum{TOF}_{Aj}}} - {TOF}_{Bj}}},} & \left( {{Equation}8} \right)\end{matrix}$which TOF_(Aj) may denote the arrival time of a photon of a coincidenceevent in respect to crystal element A, TOF_(Bj) may denote the arrivaltime of another photon of a candidate coincidence event in respect tocrystal element B, m may denote the number of coincidence eventsdetected by crystal element A and crystal element B, and j may denotethe index number of a coincidence event detected by crystal element Aand crystal element B where j=1, 2, 3, . . . , m.

In some embodiments, T_(c) may also be determined based on the timevalue at the two peaks in the histogram. For example T_(c) may becalculated by:T _(c)=(T ₁ +T ₂)/2,  (Equation 9)in which T₁ and T₂ may denote the time value at the two peaks in thehistogram as illustrated in FIG. 17 . In some embodiments, step 1607 maybe repeatedly performed to calculate n time values T_(c) for the ncrystal element pairs including the first crystal element and the nsecond crystals.

In step 1609, the time offset of the first crystal element may becalculated based on the time values T_(c) obtained in step 1607. In someembodiments, the time offset of the first crystal element may becalculated by:OTA _(i) =OTA _(i-1) +Tc _(i).  (Equation 10)

i may denote the index number of the iteration, and i=1, 2, 3, 4, . . ., n. OTA may denote the time offset of the first crystal element. Insome embodiments, OTA₁=0. The iteration may terminate based on acriterion. In some embodiments, the criterion may be the number ofiterations.

In some embodiments, the process from step 1601 to step 1609 may berepeatedly performed to calculate the time offset of a plurality ofcrystal elements in a detector ring of the PET scanner. In someembodiments, the process from step 1601 to step 1609 may be repeatedlyperformed to calculate the time offset of a plurality of detector ringsof the PET scanner.

In some embodiments, the first crystal element and n second crystalelements may be located in two detector rings of the PET scanner.

In some embodiments, TOFs acquired by the crystal elements may becalibrated based on the time offset OTA_(k), where k may denote theindex number of a crystal element. TOF may be calibrated by:TOF _(k) ′=TOF _(k) −OTA _(k).  (Equation 11)

FIG. 18 illustrates a block diagram of the radiation source adjustmentmodule 204 according to some embodiments of the present disclosure. Theradiation source adjustment module may include an acquisition unit 1801,a first assessment unit 1802, and a second assessment unit 1803.

The acquisition unit 1801 may be configured to acquire the position of aradiation source placed in the PET scanner. The first assessment unit1802 may be used to assess the axial position of the radiation source.The second assessment unit 1803 may be used to assess thecircumferential position of the radiation source.

In some embodiments, the position of the radiation source may beassessed based on a target position. The target position may include atarget axial position and a target circumferential position.

FIG. 19 is a flowchart illustrating a process for radiation sourceadjustment according to some embodiments of the present disclosure. Theradiation source may include a solid rod radiation source, a cylinderradiation phantom, a uniform water radiation source (water mixed with aradiation source) placed in a cylindrical tube, or the like, or anycombination thereof. The process for radiation source adjustment may beused to adjust the position of a radiation source placed in the PETscanner.

In step 1901, the current position of a radiation source may beobtained. The radiation source may be placed on the table 140 asdescribed in connection with FIG. 1 . The current position of theradiation source may be sent to the host computer 160 as described inconnection with FIG. 1 . In step 1903, the current position of theradiation source may be assessed based on a target position. In someembodiments, the target position may be the central point in the FOV ofthe PET scanner. In step 1905, if the current position of radiationsource is not the target position, the host computer 160 may control themovement of the table 140 based on the target position. In step 1907,the adjustment of the radiation source may be terminated if the currentposition of the radiation source is the target source.

FIG. 20 is a flowchart illustrating a process for radiation sourceadjustment according to some embodiments of the present disclosure. Instep 2001, raw data may be collected by the PET scanner. For instance,raw data ranging from 10 megabytes to 15 megabytes may be collected. Instep 2003, the axial position of a radiation source may be obtained. Instep 2005, the axial position of the radiation source may be assessedbased the target axial positon. In step 2007, the table may be adjustedto move the radiation source based on the axial target position.

In some embodiments, the scanning bore 110 as described in connectionwith FIG. 1 may be divided into a number of portions based on the numberof detector rings of the PET scanner. The division may be even such thatthe portions are of a same size. The difference for a portion may becalculated. In some embodiments, the difference for the portion may bethe difference between the prompt coincidence counts and the delaycoincidence counts of the raw data. The prompt coincidence counts mayinclude true coincidence events, random coincidence events, and scattercoincidence events. The delay coincidence counts may include randomcoincidence events. Therefore, the difference of the portion may be thecount of true coincidence events and scatter coincidence events. A firstthreshold may be set. The scanning bore of the PET scanner may bedivided into equal parts. The first difference value that may be greaterthan the first threshold may be acquired as one endpoint of theradiation source when searching from the forward equal part to themiddle of the scanning bore and the first difference value that may begreater than the first threshold may be acquired as other endpoint ofradiation source when searching from the backward equal part of thescanning bore to the middle of the PET detector, the position of theradiation source may be acquired according to the above steps asdescribed.

Merely by way of example, the PET scanner may include 96 detector rings.The scanning bore 110 may be divided into 96 portions evenly. Thedifferences of the portions may be calculated and a waveform of thedifferences may be created. The waveform indicates that the differenceof the portion in which the radiation is located may be much larger thanthe difference of the rest portions in which the radiation source is notlocated. In some embodiments, d may denote the largest difference of theportions. The first threshold t may be calculated by:t=d*0.25.  (Equation 12)

An assessment of the 96 portions from the 1^(st) portion to the 96^(th)portion may be performed. Two portions of the 96 portions may beselected for determining both ends of the radiation source if the firstthreshold is satisfied. If the two portions are the 1^(st) portion andthe 96^(th) portion, the axial position of the radiation source may bevalidated indicating the target axial position is satisfied. Otherwise,the axial target position is not satisfied and the host compute mayinstruct the table to move the radiation source based on the axialtarget position. In some embodiments, the portions may be divided basedon the length of the scanning bore. As the length of the scanning boreis knows, the length of a portion may be known as well. The radiationsource may be moved based on the length of one or more portions, forexample, the length of 1 portion, the length of 32 portions, or thelike.

If the assessment of step 2005 is validated, step 2011 may be performedand the circumferential position of the radiation source may beobtained. Otherwise, raw data may be collected in step 2009. In step2009, raw data may be collected by the PET scanner. In some embodiments,raw data ranging from 10 megabytes to 15 megabytes may be collected.

In step 2011, the circumferential position of the radiation source maybe obtained.

In some embodiments, 2m−1 sinograms may be obtained, in which m maydenote the number of detector rings of the PET scanner. For instance,the PET scanner may include 96 detector rings, and therefore, 191sinograms may be obtained.

In some embodiments, the 2m−1 sinograms may be divided into a number ofgroups evenly. For a group of the number of groups, the sinograms of thegroup may be accumulated to generate an accumulated sinogram. In someembodiments, each sinogram of the group may be denoted by atwo-dimensional (2D) array; an accumulated sinogram may be generated byadding the numbers corresponding to a same row position and a samecolumn position of the 2D arrays together. The center of the radiationsource at every angle of the accumulated sinogram may be obtained basedon the Gaussian function and the accumulated sinogram. The center of theradiation source in every angle of the accumulated sinogram may befitted based on a sine wave and be corrected based on geometry radian,to generate the circumferential position of the radiation source.

In some embodiments, the circumferential position of all the groups maybe fitted based on a straight line to acquire the center line of theradiation source. The center line of the radiation source may beassessed based on the center line of the FOV of the PET scanner.

In some embodiments, as illustrated in FIG. 21 , a sinogram may be atwo-dimensional array whose row and column may be denoted by S and Φ,respectively. S may denote the distance from the center of the FOV(corresponding to the coordinate origin O in FIG. 21 ) to the LOR, and Φmay denote the angle between LOR and the y-axis.

In some embodiments, n sinograms may be divided into t groups, and eachgroup may have d (t=n/d) sinograms. d sinograms may be accumulated ineach group, and t accumulated sinograms may be generated. For anaccumulated sinogram, the center of the radiation source at an angle maybe calculated based on Gaussian fitting. Therefore, t centers of theradiation source may be obtained. The t centers of the radiation sourcemay be fitted based on sine fitting so that t coordinates of theradiation source in the SOT coordinate system may be calculated. Thecoordinate of the radiation source may be transformed into that of thexOy coordinate system. The t coordinates may be fitted based on linefitting and the center line of the radiation source may be obtained. Thecircumferential position of the radiation source may be assessed basedon the center line.

Merely by way of example, the PET scanner having 96 detector rings. Onehundred ninety-one sinograms may be generated. The 191 sinograms may bedivided into 6 groups, and each group may have 32 sinograms, except thatthe last group may have 31 sinograms. Six accumulated sinograms may begenerated by fitting the 6 groups of sinograms based on Gaussianfitting. The centers of the radiation source at every angle of theaccumulated sinograms may be fitted based on sine fitting, calibratedbased on geometry curve correction, and 6 centers may be generated. The6 centers may be fitted based on line fitting and the center line of theradiation source may be obtained.

In step 2013, the circumferential position of the radiation source maybe assessed based on the target circumferential position. If theassessment in step 2013 validates, step 2017 may be performed and theadjustment of the radiation source may be terminated. If the assessmentin step 2013 fails, step 2015 may be performed and the table of the PETscanner may be adjusted based on the target circumferential position.

FIG. 22A and FIG. 22B illustrate the side view of an exemplary transportdevice 2210 according to some embodiments of the present disclosure. Thetransport device 2210 may include a shield 2240, an adjusting component2250 that is placed inside the shield 2240, and a base 2260 connectedthe shield 2240.

The shield 2240 may include an opening 2221. The adjusting component2250 may be controlled to adjust a radiation source 2220 in the shield2240. In some embodiments, the adjusting component 2250 may beconfigured to control the horizontal movement of the radiation source2220, for example, moving away from the opening 2221, moving close tothe opening 2221, etc.

In some embodiments, the shield 2240 may be placed on the base 2260,connected and fixed with the base 2260. In some embodiments, thetransport device 2210 may be used to calibrate the PET scanner. Throughmoving the base 2260 to adjust the position of the transport device2210, to move the radiation source 2220 to a target position. The shield2240 may reduce the amount of the radiation emission.

In some embodiments, the adjusting component 2250 may include a fixedcomponent 2251 and a pushing component 2253. The pushing component 2253may be connected to the fixed component 2251. The pushing component 2253may be configured to adjust the position of the radiation source 2220fixed on the fixed component 2251.

As illustrated in FIG. 22A and FIG. 22B, moving the radiation source2220 to the onset station. Referring to FIG. 22B, the radiation source2220 may be partially moved in a PET scanner 2230 by the transportdevice 2210. Annihilation events may occur in the portion of theradiation source 2220 located in the PET scanner 2230, and photons maybe generated. The crystal elements of the PET scanner 2230 may beconfigured to receive the photons and calculate TOF of the photons.Afterwards, the radiation source 2220 may be moved back into thetransport device 2210.

In some embodiments, the pushing component 2253 may include a slider anda driving component, the slider may be driven by the driving componentto control the movement of the radiation source fixed on the fixedcomponent 2251.

In some embodiments, the sidewall of the shield 2240 may be of a regularshape and have center lines drawn therein. The laser lamp on the lefttop and the right top of the PET scanner to position the radiationsource 2220. In some embodiments, the laser may be aligned with thecenter lines of the sidewalls of the shield 2240.

In some embodiments, a lead rail 2270 may be mounted in the inner wallof the shield 2240. The lead rail 2270 may stretch across the shield2240 horizontally from the right to the opening 2221. The lead rail 2270may be used to reduce the friction between the pushing component 2253and the inner wall of the shield 2240. The radiation source 2220 may bemoved along the lead rail 2270 by the adjusting component 2250. Forexample, the radiation source 2220 may be moved out of the shield 2240,moved back in the shield 2240, etc.

In some embodiments, at least two limit parts 2280 may be mounted on thelead rail 2270. In some embodiments, the limit part 2280 may include amechanical limit switch. The pushing component 2250 may be eitherautomatically or manually moved.

In some embodiments, at least the limit part 2280 may include anelectrical limit switch, the electrical limit switch may be used to stopthe movement of the pushing component 2150 when the pushing componentarrives at a target position.

In some embodiments, the base 2260 may include a mobile component 2265.In some embodiments, the mobile component 2265 may include wheels asillustrated in FIG. 22A and FIG. 22B. In some embodiments, the mobilecomponent 2265 may include meshing gears that may be meshed with amoving rail placed on the ground. The moving rail may be used to movethe transport device 2210.

FIG. 23A shows the front view of the transport device 2310 according tosome embodiments of the present disclosure. FIG. 23B shows the side viewof the transport device 2310 according to some embodiments of thepresent disclosure.

In some embodiments, the shield 2340 may be connected to the base 2361through a rotating component 2363. The rotating component 2363 may beused to rotate the shield 2340. In some embodiments, the base 2361 mayinclude a support and two grasp arms 2362 that may be connected to theshield 2340 via the rotating component 2363 located in the two oppositeside of the support. In some embodiments, a groove 2364 may be set onthe based 2361. The groove 2364 may be used to match the opening 2321 ofthe shield 2340. In some embodiments, the opening 2321 of the shield2430 may be an arc as illustrated in the figure.

When a radiation source is move by the transport device 2310, therotating component 2363 may be rotated either clockwise orcounter-clockwise to control the movement of the shield 2340 so that theopening 2321 of the shield 2340 may remain in the groove 2364. As aresult, the radiation source may be enclosed in the shield 2340 and maynot emit radial rays that may be harmful to an operator of the transportdevice 2310.

In some embodiments, the base 2361 may include a mobile component 2365.In some embodiments, the mobile component 2365 may include wheels asillustrated in FIG. 23A and FIG. 23B. These embodiments are non-limitingexamples, in which represent similar structures to move the transportdevice 2310 throughout the several views of the drawings. In someembodiments, the mobile component 2365 may include meshing gears thatmay be meshed with a moving rail placed on the ground. In someembodiments, the mobile component 2365 may include a fiction drive thatmay be slipped by friction wheels. In some embodiments, the mobilecomponent 2365 may include a chain drive that may drive wheels by achain. In some embodiments, the mobile component 2365 may include a beltdrive that may drive wheels by a belt.

FIG. 24 -FIG. 27 illustrate a method for time calibration for the PETscanner. A radiation source 2503 may be placed within the FOV of the PETscanner. The time offset of the PET scanner may be determined based on afirst TOF, a second TOF, and the position of the radiation source 2503.The radiation source 2053 may be moveable. A first TOF may be determinedbased on the coincidence event data for an LOR. The position of theradiation source 2503 may also be determined based on the coincidenceevent data. A second TOF may be calculated based on the position of theradiation source 2503. The time offset of the PET scanner may becalculated based on the first TOF and the second TOF. In someembodiments, the time offset may be a channel delay.

FIG. 25 illustrates an exemplary detector ring of a PET scanneraccording to some embodiments of the present disclosure. In someembodiments, the PET scanner may include multiple detector rings 2501. Adetector ring may include multiple detector units 2502. A radiationsource 2503 may be placed inside the FOV of the PET scanner.

FIG. 26 is a flowchart illustrating a process for time calibration forthe PET scanner according to some embodiments of the present disclosure.In step 2601, a radiation source may be placed within the FOV of the PETscanner. The radiation source may be used to generate coincidenceevents. A coincidence event of the coincidence events may produce twophotons. The two photons may travel along a same line but at oppositedirections. The track of the two photons may be termed as an LOR. Eachcoincidence event may correspond to a line of response. In someembodiments, the radiation source may be a hollow cylindrical radiationsource, a solid cylindrical radiation source, a linear radiation source,etc. In some embodiments, the radiation source may be symmetric. In someembodiments, the radiation source may be asymmetric. In someembodiments, the thickness of the portion of the phantom filled with theradiation source may be uniform. In some embodiments, the thickness ofthe portion of the phantom filled with the radiation source may benonuniform. In some embodiments, the size of the radiation source may bedetermined based on the size of the FOV of the PET scanner. In someembodiments, the diameter of the radiation source may range fromD_(FOV)/2 to D_(FOV), where D_(FOV) may denote the length of the FOV inthe radial direction. In some embodiments, the length of the radiationsource in the axial direction may be equal to or more than the length ofthe FOV in the axial direction. The radiation source may be placed inany suitable position in the PET scanner. In some embodiments, thecentral axis of the radiation source may be parallel to the central axisof the FOV In some embodiments, the central axis of the radiation sourcemay be nonparallel to the central axis of the FOV. In some embodiments,the radiation source may be placed at the center of the FOV. In someembodiments, the radiation source may be placed at the peripheral partof the center of the FOV.

In step 2603, TOFs of the LORs may be calculated. In some embodiments,two detector units may be selected in one detector ring 2501 to form adetector unit pair. LOR is the line connecting the two detector units2502, and the LOR may pass through the radiation source 2503. Aplurality of LORs may be determined for the PET scanner. L1, L2, L3, L4,L5, L6 may be exemplary LORs for the PET scanner as illustrated in FIG.25 .

In some embodiments, a table may be used to record the coincidence eventdata. In some embodiments, an LOR (r_(a), r_(b), i_(a), i_(b)) maydenote a coincidence event in the table, where r_(a) and r_(b) maydenote the axial position of the detector unit pair, i_(a) and i_(b) maydenote the circumferential position of the detector unit pair, andi_(a)<i_(b). The total number of LORs may be calculated by:

$\begin{matrix}{{R_{T}^{2}\frac{I_{T}\left( {I_{T} - 1} \right)}{2}},} & \left( {{Equation}12} \right)\end{matrix}$in which R_(T) may denote the total number of the detector rings in thePET scanner, and I_(T) may denote the total number of the detector unitsin a detector ring of the PET scanner.

In some embodiments, a first coordinate system (x-y) may be set up forthe detector ring as shown in FIG. 27 . The plane determined by thex-axis and the y-axis of the first coordinate system may be parallel tothe cross-section of the PET scanner, parallel to the detector ring. Asecond coordinate system (S-T) may be set up for each LOR in the xOyplane. In some embodiments, the horizontal axis of the second coordinatesystem may be parallel to the LOR. As shown in FIG. 13 , the includedangle between the first coordinate system (x-y) and the secondcoordinate system (S-T) may be Φ. In some embodiments, LOR (r_(a),r_(b), α, rad) may denote a coincidence event in a sinogram, where r_(a)and r_(b) may denote the axial position of the detector unit pair, α maydenote the included angle between an LOR and the y-axis of the firstcoordinate system (x-y), and rad may denote the distance between the LORand the center of the detector ring (O as shown in FIG. 27 ).

In some embodiment, an LOR may include a plurality of coincidenceevents, and each coincidence event may have a TOF. Therefore, the firstTOF of the LOR may be an average of all the TOFs of the coincidenceevents. In some embodiments, the first TOF of the LOR may be the timevalue at the center of the histogram determined based on all coincidenceevents occurred on the LOR.

In some embodiments, the first TOF of an LOR may be acquired based onthe histogram created based on all coincidence events occurred on theLOR. The time value of the center of the histogram may be that of thefirst TOF. In some embodiments, the first TOF may be calculated by:

$\begin{matrix}{{{\delta t^{\prime}} = \frac{\underset{i}{\Sigma}\delta t_{i}n_{i}}{\underset{i}{\Sigma}n_{i}}},} & \left( {{Equation}13} \right)\end{matrix}$in which i may denote the index number of bin of the histogram,i=−(N−1)/2, −(N−1)/2+1, . . . , 0, 1, 2, . . . , (N−1)/2, N may denotethe number of bins, δt_(i) may denote the first TOF of the i^(th) bin,and n_(i) may denote the number of coincidence events of i^(th) the bin.In some embodiments, N may be an odd number.

In step 2605, the position of the radiation source may be determined anda second TOF of the coincidence event may be calculated based on theposition. In some embodiments, the position of the radiation source maybe determined based on the reconstructed image of the radiation source.

In some embodiments, the relationship between the x-y coordinate and theS-T coordinate may be calculated by:t=y cos φ−x sin φ,  (Equation 14)in which (x,y) may denote the position of the radiation source in thex-y coordinate system, and (t, φ) may denote the position of theradiation source in the S-T coordinate system.

In some embodiments, the coordinate (x,y) of the radiation source 2503may be calculated after the coordinate (s,t) is determined.

In some embodiments, the second TOF may be determined based on thecenter of the intersection part between the LOR and the radiation source2503. As shown in FIG. 24 , the LOR L1 may pass through the radiationsource 2503. Line segment MN may denote the interaction part between theLOR and the radiation source. Q may denote the center of the linesegment MN. The TOF of the coincidence event happening at Q may bedetermined as the second TOF for L1.

In some embodiments, the distance difference between the travel lengthsof the two photons generated in a coincidence event at Q may be denotedas δl. δl may be calculated by:δl=2(y cos φ−x sin φ),  (Equation 15)in which (x_(Q), y_(Q)) may denote the coordinate of Q in the firstcoordinate system (x-y), φ may denote the including angle between L1 andthe y-axis of the first coordinate system (X-Y).

The TOF of the two photons generated in a coincidence event at Q may bedesignated as the second TOF δt. δt may be calculated by:δt=δl/c,  (Equation 16)in which c may denote the speed of light.

In step 2607, the time offset of the detector units on both ends of eachLOR may be determined based on the first TOF and the second TOF of thecoincidence event for each of a plurality of LORs. In some embodiments,the time offset may be a channel delay of the detector units. In someembodiments, the first TOF for an LOR may be denoted as δt′. The secondTOF for an LOR may be denoted as δt. The difference between δt′ and δtmay be denoted as Δ_(t). Δ_(t) may be calculated by:Δ_(t) =δt′−δt=TO _(a) −TO _(b).  (Equation 17)in which TO_(a) and TO_(b) may denote the channel delay of the twodetector units.

In some embodiments, an equation set for the channel delay may be setup. In some embodiments, the channel delay may be calculated by:

$\begin{matrix}{{{HT} = {\Delta t}},} & \left( {{Equation}18} \right)\end{matrix}$ $\begin{matrix}{{H = \begin{bmatrix}1 & {- 1} & 0 & 0 & \ldots \\0 & 1 & {- 1} & 0 & \ldots \\0 & 0 & 1 & {- 1} & \ldots \\ & \ldots & \ldots & & \end{bmatrix}},} & \left( {{Equation}19} \right)\end{matrix}$ $\begin{matrix}{{T = \begin{pmatrix}{TO}_{1} \\{TO}_{2} \\{TO}_{3} \\\ldots\end{pmatrix}},{and}} & \left( {{Equation}20} \right)\end{matrix}$ $\begin{matrix}{{\Delta = \begin{pmatrix}{{\delta t_{1}^{\prime}} - {\delta t_{1}}} \\{{\delta t_{2}^{\prime}} - {\delta t_{2}}} \\{{\delta t_{3}^{\prime}} - {\delta t_{3}}} \\\ldots\end{pmatrix}},} & \left( {{Equation}21} \right)\end{matrix}$in which H may denote a matrix of coefficients, T may denote the channeldelay of the two detector units at both ends of the LOR, and Δ_(t) maydenote the difference between the first TOF δt′ and the second TOF δt.

In some embodiments, the channel delay may be obtained through aniteration process for the two detector units at both ends of the LOR.The iteration process may be terminated based on a criteria. In someembodiments, the iterative process may terminated based on the number ofiterations. In some embodiments, the iterative process may terminatedwhen the channel delay is less than a threshold.

The relative location of the first detector unit and the correspondingsecond detector unit is determined based on the dimension of the imagedreconstructed by the PET scanner. For a two-dimensional imagereconstruction, the first detector unit and the second detector unitsmay be located in a same detector ring. For a three-dimensional imagereconstruction, the first detector unit and the second detector unitsmay not be located in a same detector ring.

In step 2609, the detector unit may be calibrated based on the channeldelay calculated in step 2607. In some embodiments, the raw dataacquired by the detector unit in practical use may be corrected based onthe channel delay. In some embodiments, the channel delay calculated instep 2607 may be stored in the storage of the PET scanner. In someembodiments, the raw data acquired in practical use may be stored. Andthe channel delay may be used in the image reconstruction process.

Having thus described the basic concepts, it may be rather apparent tothose skilled in the art after reading this detailed disclosure that theforegoing detailed disclosure is intended to be presented by way ofexample only and is not limiting. Various alterations, improvements, andmodifications may occur and are intended to those skilled in the art,though not expressly stated herein. These alterations, improvements, andmodifications are intended to be suggested by this disclosure, and arewithin the spirit and scope of the exemplary embodiments of thisdisclosure.

Moreover, certain terminology has been used to describe embodiments ofthe present disclosure. For example, the terms “one embodiment,” “anembodiment,” and/or “some embodiments” mean that a particular feature,structure or characteristic described in connection with the embodimentis included in at least one embodiment of the present disclosure.Therefore, it is emphasized and should be appreciated that two or morereferences to “an embodiment” or “one embodiment” or “an alternativeembodiment” in various portions of this specification are notnecessarily all referring to the same embodiment. Furthermore, theparticular features, structures or characteristics may be combined assuitable in one or more embodiments of the present disclosure.

Further, it will be appreciated by one skilled in the art, aspects ofthe present disclosure may be illustrated and described herein in any ofa number of patentable classes or context including any new and usefulprocess, machine, manufacture, or composition of matter, or any new anduseful improvement thereof. Accordingly, aspects of the presentdisclosure may be implemented entirely hardware, entirely software(including firmware, resident software, micro-code, etc.) or combiningsoftware and hardware implementation that may all generally be referredto herein as a “block,” “module,” “engine,” “unit,” “component,” or“system.” Furthermore, aspects of the present disclosure may take theform of a computer program product embodied in one or more computerreadable media having computer readable program code embodied thereon.

A computer readable signal medium may include a propagated data signalwith computer readable program code embodied therein, for example, inbaseband or as part of a carrier wave. Such a propagated signal may takeany of a variety of forms, including electro-magnetic, optical, or thelike, or any suitable combination thereof. A computer readable signalmedium may be any computer readable medium that is not a computerreadable storage medium and that may communicate, propagate, ortransport a program for use by or in connection with an instructionexecution system, apparatus, or device. Program code embodied on acomputer readable signal medium may be transmitted using any appropriatemedium, including wireless, wireline, optical fiber cable, RF, or thelike, or any suitable combination of the foregoing.

Computer program code for carrying out operations for aspects of thepresent disclosure may be written in any combination of one or moreprogramming languages, including an object oriented programming languagesuch as Java, Scala, Smalltalk, Eiffel, JADE, Emerald, C++, C#, VB. NET,Python or the like, conventional procedural programming languages, suchas the “C” programming language, Visual Basic, Fortran 2003, Perl, COBOL2002, PHP, ABAP, dynamic programming languages such as Python, Ruby andGroovy, or other programming languages. The program code may executeentirely on the user's computer, partly on the user's computer, as astand-alone software package, partly on the user's computer and partlyon a remote computer or entirely on the remote computer or server. Inthe latter scenario, the remote computer may be connected to the user'scomputer through any type of network, including a local area network(LAN) or a wide area network (WAN), or the connection may be made to anexternal computer (for example, through the Internet using an InternetService Provider) or in a cloud computing environment or offered as aservice such as a Software as a Service (SaaS).

Furthermore, the recited order of processing elements or sequences, orthe use of numbers, letters, or other designations therefore, is notintended to limit the claimed processes and methods to any order exceptas may be specified in the claims. Although the above disclosurediscusses through various examples what is currently considered to be avariety of useful embodiments of the disclosure, it is to be understoodthat such detail is solely for that purpose, and that the appendedclaims are not limited to the disclosed embodiments, but, on thecontrary, are intended to cover modifications and equivalentarrangements that are within the spirit and scope of the disclosedembodiments. For example, although the implementation of variouscomponents described above may be embodied in a hardware device, it mayalso be implemented as a software only solution—e.g., an installation onan existing server or mobile device.

Similarly, it should be appreciated that in the foregoing description ofembodiments of the present disclosure, various features are sometimesgrouped together in a single embodiment, figure, or description thereoffor the purpose of streamlining the disclosure aiding in theunderstanding of one or more of the various inventive embodiments. Thismethod of disclosure, however, is not to be interpreted as reflecting anintention that the claimed subject matter requires more features thanare expressly recited in each claim. Rather, inventive embodiments liein less than all features of a single foregoing disclosed embodiment.

In some embodiments, the numbers expressing quantities of ingredients,properties such as molecular weight, reaction conditions, and so forth,used to describe and claim certain embodiments of the application are tobe understood as being modified in some instances by the term “about,”“approximate,” or “substantially.” For example, “about,” “approximate,”or “substantially” may indicate ±20% variation of the value itdescribes, unless otherwise stated. Accordingly, in some embodiments,the numerical parameters set forth in the written description andattached claims are approximations that may vary depending upon thedesired properties sought to be obtained by a particular embodiment. Insome embodiments, the numerical parameters should be construed in lightof the number of reported significant digits and by applying ordinaryrounding techniques. Notwithstanding that the numerical ranges andparameters setting forth the broad scope of some embodiments of theapplication are approximations, the numerical values set forth in thespecific examples are reported as precisely as practicable.

Each of the patents, patent applications, publications of patentapplications, and other material, such as articles, books,specifications, publications, documents, things, and/or the like,referenced herein is hereby incorporated herein by this reference in itsentirety for all purposes, excepting any prosecution file historyassociated with same, any of same that is inconsistent with or inconflict with the present document, or any of same that may have alimiting affect as to the broadest scope of the claims now or laterassociated with the present document. By way of example, should there beany inconsistency or conflict between the description, definition,and/or the use of a term associated with any of the incorporatedmaterial and that associated with the present document, the description,definition, and/or the use of the term in the present document shallprevail.

In closing, it is to be understood that the embodiments of theapplication disclosed herein are illustrative of the principles of theembodiments of the application. Other modifications that may be employedmay be within the scope of the application. Thus, by way of example, butnot of limitation, alternative configurations of the embodiments of theapplication may be utilized in accordance with the teachings herein.Accordingly, embodiments of the present application are not limited tothat precisely as shown and described.

What is claimed is:
 1. A method for calibrating a PET scanner, the PETscanner having a plurality of detector rings, each detector ring havinga plurality of detector units, the method comprising: determining a lineof response (LOR) connecting a first detector unit and a second detectorunit of the PET scanner, wherein the LOR correlates to a plurality ofcoincidence events resulting from annihilation of positrons emitted by aradiation source placed at a position in a field of view (FOV) of thePET scanner, each of the plurality of coincidence events is detected bythe first detector unit and the second detector unit, and each of theplurality of coincidence events has a time of flight (TOF); determininga first TOF of the LOR, the first TOF being an average of the pluralityof TOFs of the plurality of coincidence events; determining a second TOFof the LOR by dividing a distance difference between a first distance ofthe radiation source with respect to the first detector unit and asecond distance of the radiation source with respect to the seconddetector unit by the speed of light; and calculating a differencebetween the first TOF and the second TOF as a channel delay between thefirst detector unit and the second detector unit.
 2. The method of claim1, wherein an axis of the radiation source is parallel to a longitudinalaxis of the PET scanner.
 3. The method of claim 1, wherein thedetermining a first TOF of the LOR comprises: creating a histogramcomprising the plurality of TOFs of the plurality of coincidence events;calculating a time value of the center of the histogram; and designatingthe time value of the center of the histogram as the first TOF of theLOR.
 4. The method of claim 1, wherein the radiation source is wrappedby a hollow cylindrical phantom.
 5. The method of claim 4, wherein thehollow cylindrical phantom is placed at a center of the FOV.
 6. Themethod of claim 1, further comprising: determining whether the positionof the radiation source is a target position; determining that theposition is not the target position; and adjusting a table where theradiation source is placed to move the radiation source to the targetposition.
 7. The method of claim 6, wherein the determining whether theposition of the radiation source is a target position comprises:determining whether an axial position of the radiation source is at atarget axial position; and determining whether a circumferentialposition of the radiation source is at a target circumferentialposition.
 8. A positron emission tomography (PET) system having aplurality of detector rings, and each detector ring having a pluralityof detector units, the PET system comprising: a PET scanner; acoincidence event detection circuit for detecting coincidence eventsresulting from annihilation of positrons emitted by a radiation sourceplaced at a position in a field of view (FOV) of the PET scanner; and ahost computer that is configured to perform operations comprising:determining a line of response (LOR) connecting a first detector unitand a second detector unit of the PET scanner, wherein the LORcorrelates to a plurality of coincidence events, each of the pluralityof coincidence events is detected by the first detector unit and thesecond detector unit, and each of the plurality of coincidence eventshas a time of flight (TOF); determining a first TOF of the LOR, thefirst TOF being an average of the plurality of TOFs of the plurality ofcoincidence events; determining a second TOF of the LOR by dividing adistance difference between a first distance of the radiation sourcewith respect to the first detector unit and a second distance of theradiation source with respect to the second detector unit by the speedof light; and calculating a difference between the first TOF and thesecond TOF as a channel delay between the first detector unit and thesecond detector unit.
 9. The PET system of claim 8, wherein an axis ofthe radiation source is parallel to a longitudinal axis of the PETscanner.
 10. The PET system of claim 8, wherein the determining a firstTOF of the LOR comprises: creating a histogram comprising the pluralityof TOFs of the plurality of coincidence events; calculating a time valueof the center of the histogram; and designating the time value of thecenter of the histogram as the first TOF of the LOR.
 11. The PET systemof claim 8, wherein the radiation source is wrapped by a hollowcylindrical phantom.
 12. The PET system of claim 11, wherein the hollowcylindrical phantom is placed at a center of the FOV.
 13. The PET systemof claim 8, wherein the host computer is further configured to performthe operations comprising: determining whether the position of theradiation source is a target position; determining that the position isnot the target position; and adjusting a table where the radiationsource is placed to move the radiation source to the target position.14. The PET system of claim 13, wherein the determining whether theposition of the radiation source is a target position comprises:determining whether an axial position of the radiation source is at atarget axial position; and determining whether a circumferentialposition of the radiation source is at a target circumferentialposition.
 15. A non-transitory computer-readable medium embodying acomputer program product, the computer program product comprisinginstructions for calibrating a PET scanner, the PET scanner having aplurality of detector rings, each detector ring having a plurality ofdetector units, wherein the instructions are configured to cause acomputing device to perform operations including: determining a line ofresponse (LOR) connecting a first detector unit and a second detectorunit of the PET scanner, wherein the LOR correlates to a plurality ofcoincidence events resulting from annihilation of positrons emitted by aradiation source placed at a position in a field of view (FOV) of thePET scanner, each of the plurality of coincidence events is detected bythe first detector unit and the second detector unit, and each of theplurality of coincidence events has a time of flight (TOF); determininga first TOF of the LOR, the first TOF being an average of the pluralityof TOFs of the plurality of coincidence events; determining a second TOFof the LOR by dividing a distance difference between a first distance ofthe radiation source with respect to the first detector unit and asecond distance of the radiation source with respect to the seconddetector unit by the speed of light; and calculating a differencebetween the first TOF and the second TOF as a channel delay between thefirst detector unit and the second detector unit.
 16. The non-transitorycomputer-readable medium of claim 15, wherein an axis of the radiationsource is parallel to a longitudinal axis of the PET scanner.
 17. Thenon-transitory computer-readable medium of claim 15, wherein thedetermining a first TOF of the LOR comprises: creating a histogramcomprising the plurality of TOFs of the plurality of coincidence events;calculating a time value of the center of the histogram; and designatingthe time value of the center of the histogram as the first TOF of theLOR.